The global prevalence and associated risk factors of Eimeria infection in domestic chickens: A systematic review and meta‐analysis

Abstract Background Eimeria is a protozoan parasite that affects poultry, particularly chickens, causing a disease known as coccidiosis. This disease imposes substantial significant economic challenges to the poultry sector. Objectives The current study aimed to estimate the global prevalence and associated risk factors of Eimeria in domestic chickens. Methods Multiple databases (Scopus, PubMed, ProQuest, Web of Science and Google Scholar) were searched for articles published until June 2023. The pooled prevalence was estimated using a random‐effects model with a 95% confidence interval. The statistical analysis was conducted using meta packages in R version (3.6.1). Results In total, 41 articles fulfilled the eligibility criteria. The global pooled prevalence was 44.3% (36.9%–51.8%) with Eimeria tenella (38.7%, 30.1%–47.7%) as the most prevalent species. The highest pooled prevalence was related to the Western Pacific Region (80.5%, 72.6%–87.3%) and urban areas (44.4%, 36.5%–52.6%). Moreover, areas with humid subtropical climates represent the highest overall prevalence (75.8%, 46.6%–95.9%). Conclusion The necessity for robust and innovative strategies for preventing and managing this disease cannot be overstated. Addressing Eimeria impact is crucial not only for safeguarding poultry health but also for sustaining the economic viability of the poultry industry.


INTRODUCTION
Coccidiosis is recognized as the parasitic disease with the highest economic impact on the poultry production industry (Allen & Fetterer, 2002).A wide range of domestic and wild animal species, including cattle, sheep, goats, pigs, chickens, turkeys, rabbits, reptiles and fish, are affected by coccidiosis, which is the most potent cause of economic losses, especially regarding poultry and livestock farming (Ahmad et al., 2016;Jenkins, 2001).The disease affects chickens, with infection rates reaching 5% and 20% for clinical and sub-clinical cases, respectively.
The principal causative agents of coccidiosis in poultry are attributed to members of the Eimeria genus, which are obligatory intracellular protozoan parasites of the apicomplexan class, classified within the family Eimeriidae (Peek & others, 2010;Shirley et al., 2005).
In recent years, concerns over the impact of infections caused by Eimeria species on the livestock and poultry industries have gained increasing attention on a global scale (Almeria et al., 2020;Kadykalo et al., 2018;Peek & Landman, 2011).
There are seven different species of Eimeria in chickens that mature within the intestinal epithelial cells of the definitive host (Blake & Tomley, 2014;Jenkins, 2001;McDonald & Shirley, 2009).Among different species, Eimeria tenella, Eimeria acervulina and Eimeria maxima impose substantial economic losses in poultry industry (El-Ghany, 2020).
Eimeria parasites have exogenous and endogenous life cycle.The exogenous phase involves the release of oocysts into the environment, whereas the endogenous phase occurs within the host's intestine, where the parasite undergoes asexual (schizogony) and sexual reproduction (gametogeny) (McDonald & Shirley, 2009).The infection can be potentially transmitted through various routes, such as direct birdto-bird contact, clothing and footwear, insects like flies and beetles, as well as wild birds that come into contact with poultry facilities, breeding equipment and non-sterilized contaminated feed bags.In addition, climatic conditions have a significant impact on transmission, as warm and humid tropical climates are potential contributors to the occurrence of these parasites (Abdelrahman et al., 2014;Etuk et al., 2004;Serbessa et al., 2023;Toledo et al., 2011).
The infection is commonly associated with severe intestinal conflicts, diarrhoea, dehydration, weight loss, lack of appetite and weakness.It significantly impacts animal farming by reducing production and increasing mortality rates (Boulton et al., 2018;El-Shazly et al., 2020;Zhou et al., 2020b).
Adopting effective methods of caring for animals and using preventive treatments like various types of anticoccidial medications (such as sulphonamides, ionophores and toltrazuril) that work against different parts of the parasite's life cycle are the recommended approaches to avoid the disease, especially in the field of poultry farming (Chartier & Paraud, 2012;McDonald & Shirley, 2009).
The current study aims to enhance our understanding of the global prevalence of Eimeria parasites in chickens by thoroughly analysing the available studies, which help to improve practices concerning poultry care and provide valuable insights for future research.

Search strategy
This study complies with the preferred reporting items for systematic reviews and meta-analysis checklist (Page et al., 2021).We conducted Additionally, the reference list was manually searched to identify additional relevant studies that were not retrievable through a database search.Two authors independently conducted searches, assessed titles and abstracts, and reviewed the full texts of the papers.

Inclusion and exclusion criteria
Full-text articles were considered eligible if they satisfied the inclusion criteria outlined as follows:  1 & 2).

Quality assessment
The assessment of study quality was carried out through the utilization of the Newcastle-Ottawa Scale (as described in Table S1) (Abdoli et al., 2024;Eslahi et al., 2023).The scoring process included three components: (1) selection (maximum of 5 stars), ( 2) comparability (maximum of 2 stars) and ( 3) outcome (maximum of 3 stars), each with its respective score ranges.

Data synthesis and statistical analysis
To estimate the pooled prevalence of Eimeria parasites in domestic chickens, a Freeman-Tukey double arcsine transformation was applied using a random-effects model with a 95% confidence interval (95%CI).
In order to specify the impact of the year of publication on the prevalence, a meta-regression analysis was applied.
We employed Egger's and Begg's tests to identify potential publication bias.Additionally, publication bias was assessed using the Luis Furuya-Kanamori (LFK) index and the Doi plot (Barendregt & Doi, 2016).An LFK index within the range of outside ±2, ±2 and ±1 indicated significantly/major asymmetrical, slightly/minor asymmetrical and asymmetrical symmetrical (absence of publication bias), respectively.Furthermore, we assessed the degree of heterogeneity among the included studies using Cochrane's Q test and the inconsistency index (I 2 statistics), considering I 2 values of 0%-25% as low, 25%-50% as moderate and 50%-75% as high heterogeneity (Higgins et al., 2003).
A p-value less than 0.05 was specified as statistically significant.All procedures of statistical analyses were performed via meta and metasens packages in R version (3.6.1)(R Core Team, 2020).

Characteristics of included studies
A total of 12,870 records were retrieved from databases (651 from Scopus, 946 from PubMed, 372 from ProQuest, 501 from Web of Science and 10,400 from Google Scholar).To determine the eligibility of the literature, a selection process identified 112 full-text publications.
Among these, 8 studies lacked sufficient data, 5 studies presented overlapping data, 9 studies were composed of case reports and case series, and an additional 46 documents did not contain original data, including reviews, letters, theses or workshop materials.Consequently, after a rigorous evaluation based on critical assessment criteria, a total of 41 articles were included in the current meta-analysis (Figure 1).The estimated pooled global prevalence of Eimeria parasites in domestic chickens was 44.3% (95%CI: 36.9%-51.8%)(Figure 2).
The estimations based on WHO regions ranged from 30.8% to  2).
In terms of season, the highest pooled prevalence rate of Eimeria parasites has been found to be related to autumn (58.6%, 95%CI: 40.5%-75.6%)(Table 2).
According to the estimations based on diagnostic techniques, the studies that employed of polymerase chain reaction (PCR) method represented the highest pooled prevalence, with a rate of 68.2% (95%CI: 50.9%-83.1%)(Table 2).
In terms of the type of region (rural/urban), the highest global prevalence of Eimeria parasites was observed in samples collected from chickens in urban regions, with a rate of 44.4% (95%CI: 36.5%-52.6%)(Table 2).

Publication bias
As revealed by the results of Egger's regression test (t = 1.07, p = 0.285) and Begg's regression test (t = 1.81, p = 0.078), there were no significant indications of publication bias.Furthermore, the assessment of the Doi plot unveiled a minor asymmetry (LFK index: 1.69) (Figure 3).Abbreviations: CI, confidence interval; PCR, polymerase chain reaction.

Meta-regression
The meta-regression analysis indicates that there was a statistically significant correlation between prevalence and year of publication (slop: 19.93, p < 0.05) (Figure 4).
Based on the included papers, a map was created using the QGIS3 program (available at https://qgis.org/en/site/) to show the prevalence of Eimeria infection in domestic chickens in different parts of the world (Figure 5).

DISCUSSION
Chickens play a crucial role in food production, but the rapid worldwide spread of chicken coccidiosis is a significant threat to the poultry industry (Attree et al., 2021).Epidemiological investigations on the occurrence of Eimeria species serve as valuable instruments for the prevention and management of coccidiosis (Morris & Gasser, 2006;Ogedengbe et al., 2011).To the best of our knowledge, this study is the first systematic review and meta-analysis regarding the prevalence of Eimeria infection in domestic chickens.
The results of the current study revealed a global pooled prevalence of 44.3% with E. tenella being the most prevalent species (38.7%).
Among Eimeria parasites affecting chickens, E. tenella is regarded as the most critical species due to its high virulence, causing severe haemorrhage and high mortality rates (Fossum et al., 2009).It infects and lives in intestinal epithelial cells, and during its replication cycle, the para-site causes damage to the epithelial cells, which leads to haemorrhagic diarrhoea and negatively impacts growth performance and intestinal health in broiler chickens (Zaman et al., 2012).
The oocysts of E. tenella remained viable for 32 days when exposed to 61% humidity and for 49-52 days when subjected to 90% humidity (Fayer, 1980;Gajadhar & Allen, 2004).In the poultry industry, the prevalence of this parasite among chickens resulted in significant economic losses due to expenses associated with prevention and treatment, amounting to over $3.5 billion every year (Blake & Tomley, 2014).Numerous studies have demonstrated that the adverse impacts of infection caused by E. tenella in broiler chickens are associated with alterations in the cecal microbiota (Macdonald et al., 2017;Martynova-Van Kley et al., 2012;Zhou, et al., 2020a).
Nevertheless, even non-pathogenic species of Eimeria have economic significance as their infections can lead to notable declines in feed efficiency, weight gain and overall productivity, even in apparently healthy birds (Morris et al., 2007).
This review highlights that Eimeria infections in domestic chickens are most prevalent in regions characterized by a humid subtropical climate, consistent with previous reports indicating that these parasites are endemic in many tropical and subtropical areas worldwide.The presence of a humid and temperate climate provides favourable conditions for the development and persistence of protozoan cysts and/or oocysts in water or the environment (Abebe & Gugsa, 2018;Ahmed et al., 2018).
The optimal conditions for the sporulation of Eimeria oocysts are considered to be around 25 remain viable in temperatures as low as 4 • C (Anderson et al., 1976;Fayer, 1980).
Eimeria oocysts thrive in moist conditions found within poultry houses, which can be due to factors like water spillages or heavy rainfall, leading to humidity levels exceeding 60%.This is particu- larly relevant in open-house poultry rearing, a method widely used in tropical and subtropical regions and backyard production setups.
Our investigation with regard to different seasons uncovered that the highest occurrence of Eimeria infection was in autumn, which can be linked to the preference of oocysts to proceed with the sporulation and its survival during and shortly after rainy seasons in tropical areas.This trend has been observed with a heightened incidence of Eimeria infection in regions like Egypt during the winter months (coinciding with the December-February rainy season), Ethiopia following the October rains and the Kashmir valley in India from September to November (Attree et al., 2021).
However, it is worth noting that higher temperatures can have an quantitative real-time PCR to identify and quantify various Eimeria species in individual samples allowing the assessment of stored samples over extended periods.Additionally, to differentiate Eimeria species, non-quantitative PCR assays are developed (Morris et al., 2007;Velkers et al., 2010).
In line with expectations, our analyses based on different WHO regions showed that the Western Pacific Region represents the highest prevalence of Eimeria infections.This region, with 37 countries, is characterized by a significant incidence of neglected tropical diseases.
Despite substantial efforts at mass drug administration, parasitic infections persist as a public health concern in the Western Pacific region, largely due to the challenging environmental conditions prevalent in the area (Eslahi et al., 2022).
Eimeria infections in chickens are a widespread issue that occurs worldwide.Nevertheless, the prevalence of these infections can greatly differ, and several factors contribute to these variations.
These factors may include the population density at which the chickens are raised, the overall health and immune status of the animals, the quality of their nutrition and management practices, the specific breeding techniques employed, the age of the chickens, prevailing climate conditions and the size of the flock (Sun et al., 2023).
The findings from this systematic review and meta-analysis must be interpreted with caution, considering certain limitations.First, our analyses might have been influenced by publication bias, stemming from either the absence or the limited number of studies available from certain geographical regions.Second, there were various individual case reports related to Eimeria species that were not encompassed in the current study.Another limitation is that our study exclusively focused on English-language publications.Last but not least is that some of the studies included in our analyses exhibited small-study effects, which can be attributed to factors such as a limited sample size and the absence of a highly sensitive diagnostic technique.Despite these limitations, it is important to acknowledge that this study offers the most comprehensive insights into the prevalence of Eimeria infection in domestic chickens from a global standpoint.

CONCLUSION
Coccidiosis in chickens is a common and potentially serious disease caused by protozoan parasites of the genus Eimeria.It can lead to various health issues, including diarrhoea, weight loss, decreased egg production and even death in severe cases.Preventing and managing coccidiosis in poultry is crucial for maintaining the overall health and productivity of the flock.This can be achieved through measures, such as proper hygiene, vaccination and the use of coccidiostats (medications that inhibit the growth of Eimeria).Regular monitoring and early intervention are essential to minimize the impact of coccidiosis on chicken populations.
inhibitory effect, limiting the replication of the parasite.A clear example of this occurrence can be seen in Pakistan, where the highest prevalence of coccidiosis was identified at the end of the monsoon season, a period characterized by a decrease in ambient temperatures to around 25 • C(Awais et al., 2012).The highest pooled prevalence was associated with studies employing PCR technique.The diagnostic approach for coccidiosis typically involves enumerating oocysts in pooled faecal samples or litter, although this method has limitations, such as the restriction on the number of samples processed and the challenging microscopic differentiation of Eimeria species.Therefore, the necessity for improved diagnostic efficiency and quality in this regard is an utmost(Velkers et al., 2010).To mitigate the shortcomings of traditional methods, various PCR-based detection techniques have been developed(Morris et al., 2007;Velkers et al., 2010).One approach involves utilizing F I G U R E 2 Forest plots for random-effects meta-analysis of Eimeria infection in domestic chickens based on included studies (The boxes indicate the effect size of the studies (prevalence) and the whiskers indicate its confidence interval for corresponding effect size.There is no specific difference between white and black bars, only studies with a very narrow confidence interval are shown in white.In the case of diamonds, their size indicates the size of the effect, and their length indicates confidence intervals).F I G U R E 3 Eggers funnel plot and beggs funnel plot to assess publication bias inEimeria infection in domestic chicken based on included studies (a and b), coloured circles represent each study (coloured circles represent each study.The middle line is the effect size and the other two lines are the corresponding confidence ranges).Doi plot for the global prevalence of Eimeria infection in domestic chicken (c), a Luis Furuya-Kanamori (LFK) index l.69 indicates minor asymmetry.F I G U R E 4 A meta-regression graph for the global prevalence of Eimeria infection in domestic chickens based on year of publication (the pink line is the regression line, which was plotted based on the intercept and the slope of the regression model).The different coloured bubbles represent the countries under study, and their sizes indicate the effect size of each study.F I G U R E 5 The global prevalence of Eimeria infection in domestic chickens in different geographical regions of the world based on included studies (https://qgis.org/en/site/)(a).The circular plot demonstrated the diversity of Eimeria species found in the countries under consideration (b).
Main characteristics of the included studies reporting the prevalence of Eimeria in domestic chicken.Sub-group analysis based on climate, seasons, diagnostic method, humidity, longitude, latitude, sample type, type of region (rural/urban), WHO regions, annual precipitation, annual rainfall and average temperature in included studies.
TA B L E 1 • C, although these oocysts can endure and Identification Records idenƟfied through English (Scopus:651, PubMed:946, ProQuest: : 372, Web of Science: :501, and Google Scholar:10,400) databases (n=12,870) F I G U R E 1 Flow diagram of the study design process.